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of hydrogen,1 and after about 5 minutes the flask is gently heated to com-
plete the action, which may occupy 15-30 minutes, according to the nature
of the metal.

The hydrogen in the apparatus is then displaced by carbon dioxide.
The bottle B1 is then emptied and washed into E and an exactly measured
volume of the iodine solution (10, 20 or 30 c.c. according to the amount of
the precipitate) added. After some time, during which the flask is occa-
sionally shaken, the excess of iodine is titrated with the thiosulphate solution
in the usual way.

Example : 10 c.c. of iodine solution were taken and n c.c. of thiosulphate
solution were required to decolorise the excess of iodine. The sulphur has there-
fore absorbed iodine equivalent to 1511 =4 c.c. of thiosulphate, the amount
of sulphur thus being o-coo66 X 4, and the percentage of sulphur in the sample
(4 X 0-00066 x 100) ~ 5 =0-053.

This method gives excellent results and is simple to execute. It is especially
advantageous when determinations have to be made regularly, since, when
the apparatus is fitted up and the solutions ready, a determination can be made
in about 30 minutes.

6.  Determination of the Arsenic

Arsenic may be determined fairly simply by distilling as trichloride
and titrating the distillate with iodine solution.

Reagents :   (i) Solution of iodine in potassium iodide.2

2. Starch paste.

Titmtion of the iodine solution. According to Treadwell, the titre of
the iodine solution with respect to arsenic is ascertained by dissolving, in
the hot, 1-32 gram of arsenious acid (puriss.) in the least possible quantity
of concentrated caustic soda solution, the liquid being transferred quan-
titatively to a litre flask, and a drop of phenolphthalein and sufficient dilute
sulphuric acid to decolorise the liquid being added.

About 20 grams of sodium bicarbonate are dissolved in 500 c.c. of cold
water and the solution filtered and added to that of the arsenious anhydride.
If the liquid is still red, a few drops of dilute sulphuric acid are added and
the volume made up to I litre : i c.c. of this solution contains I m. grm.
of arsenic. Into 50 c.c. of this solution (0-050 gram arsenic) mixed with
a little starch paste, the iodine solution is run from a burette until the liquid
becomes blue ; the amount of arsenic corresponding with i c.c. of the
iodine solution is then calculated.

Procedure. Into a distillation flask (about |- litre) furnished with a
lateral bulb-tube are introduced 5 grams of the sample, 3-5 grams of powdered
potassium chlorate and then, gradually and with shaking and cooling, 80

1   Some authors dispense with the current of hydrogen during the attack of the
metal and merely drive the air from the apparatus before the action and displace the
pr oducts of the reaction when the metal is completely dissolved, by means of a current
of   carbon dioxide.

2  The iodine solution used for the volumetric determination of the sulphur (see p.
177) is suitable ;  or 3-3858 grams of iodine and 10 grams of potassium iodide may be
dissolved to i litre, i c.c, of this solution corresponding with i m. grm. of arsenic,iodine number of fatty